This invention relates to AM radio receivers which receive an amplitude modulated signal to provide an audio signal.
A conventional AM receiver extensively employs a superheterodyne system such as shown in FIG. 1. Referring to FIG. 1, an amplitude-modulated signal (of a frequency fs) received by an antenna 1 is amplified by a high-frequency amplifier 2, and the signal thus amplified is applied to a frequency converter 3 where it is converted into a signal having a difference frequency between the frequency fs and the oscillation frequency fl of a local oscillator 4 (or an intermediate frequency fi=fl-fs, or fs-fl). Then, the signal is amplified by an intermediate-frequency amplifier 5, and the signal thus amplified is detected by a demodulator 6 to provide an audio signal.
The conventional superheterodyne system suffers from the following drawbacks: In the case where, for instance, the local oscillation frequency fl is set to be lower than the received signal frequency fs, only a signal having a frequency higher by the intermediate frequency fi than the local-oscillation frequency fl should be received. However, a signal having a frequency lower by the intermediate frequency fi than the local oscillation frequency fl is also converted into the intermediate frequency fi if this signal is sufficiently strong, as a result of which a so-called image-frequency interference occurs; that is, the occurrence of radio interference cannot be avoided.
Furthermore, a high selectivity is required for the intermediate-frequency amplifier stage. However it is also desirable not to make the selected bandwidth too narrow, otherwise the fidelity of a signal transmission in the selected bandwidth is degraded. Therefore, it is necessary to provide an intermediate-frequency filter comprising a special collective band-pass filter, a crystal filter, or a mechanical filter, which leads an increment in the manufacturing cost.